1. Field of the Invention
This invention relates to polyorganosiloxane compositions that are curable to produce pressure sensitive adhesives and to methods of making the cured compositions. This invention also relates to articles comprising a support carrying the cured compositions of this invention.
2. Description of the Prior Art
The term, pressure sensitive adhesive, as used herein refers to adhesives that can be adhered to a surface and yet can be stripped from said surface without transferring more than trace quantities of adhesive to the surface, and can be readhered to the same or another surface because the ahdesive retains some or all of its tack and adhesive strength.
Pressure sensitive adhesives, hereinafter also referred to as PSAs, comprising polyorganosiloxanes are not new. Various mixtures of a benzene soluble resin copolymer comprising EQU R.sub.3 SiO.sub.1 /.sub.2 units and SiO.sub.4 /.sub.2 units,
hereinafter also referred to as M and Q respectively, and a hydroxyl endblocked polydiorganosiloxane have been disclosed which have PSA characteristics. U.S. Pat. Nos. 2,736,721; 2,814,601; 2,857,356; 3,528,940 and British Patent Specification No. 998,232 teach such mixtures. However, all said mixtures, whether cured or not, either require large amounts of solvent for the purpose of reducing the viscosity of the PSA to a manageable level or they display inferior adhesive strength for many applications or they do not yield PSAs which have adequate resistance to creep at high temperature or they suffer from a combination of these shortcomings.
When the prior art mixtures of a low viscosity, vinyl endblocked polydiorganosiloxane or a hydroxyl free polydiorganosiloxane and a benzene soluble resin copolymer comprising EQU R.sub.3 SiO.sub.1 /.sub.2 units and SiO.sub.4 /.sub.2 units,
are cured, PSAs are not obtained. U.S. Pat. Nos. 3,057,469; 3,284,406; 3,839,075, and British Patent Specification No. 972,594 teach such mixtures to be tack free potting compositions, primer compositions for a known PSA, vibration damping compositions and flexible casting resins.
The PSA art has long felt the need for a polyorganosiloxane PSA having high tack, good adhesive strength, excellent resistance to creep, even at high temperature and low solvent content. It is unexpected to obtain these qualities by combining certain components in critical combinations and curing the resulting mixture.